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A-a = PAO₂ − PaO₂
Normal: < 10 mmHg in young adults; < (age/4 + 4) mmHg age-adjusted; < 25 mmHg on supplemental O₂
The A-a gradient measures the difference between the oxygen tension in the alveoli (PAO₂) and the oxygen in the arterial blood (PaO₂). An elevated gradient indicates a gas exchange problem: the alveolus is full of oxygen but it is not transferring efficiently into the blood. A normal gradient with hypoxemia points to hypoventilation or low inspired O₂ as the cause.
Calculate in any patient with hypoxemia to determine the mechanism. Elevated A-a = intrapulmonary problem (V/Q mismatch, shunt, diffusion impairment). Normal A-a = extrapulmonary problem (hypoventilation, low FiO₂, high altitude).
→ 8 mmHg
Normal A-a gradient — no gas exchange impairment
→ 45 mmHg
Markedly elevated — dead-space V/Q mismatch from PE
→ 17 mmHg
Mildly elevated — some V/Q mismatch with baseline hypoventilation
Elevated A-a gradient is seen in pneumonia, PE, ARDS, pulmonary edema, atelectasis, and any condition causing V/Q mismatch or shunt. A-a gradient increases with age and with higher FiO₂. The gradient should improve with supplemental O₂ in V/Q mismatch but will NOT improve with pure shunt (blood bypasses ventilated alveoli entirely).